Now consider: (let ((var ...)) ... (hash-ref hash key (lambda () var))
Do I understand well that this thunk is not cq cannot be compiled to a constant? When compiling the thunk, the location of var relative to the top of the stack is known, is it not? Thanks, Jos > -----Original Message----- > From: Matthew Flatt [mailto:mfl...@cs.utah.edu] > Sent: 18 July 2010 22:43 > To: Jos Koot > Cc: 'Robby Findler'; 'plt-scheme' > Subject: Re: [racket] hash-ref > > Actually, you're right. A `lambda' form like > > (lambda () #f) > > that doesn't close over anything will compile to a constant. > > There is some function-call overhead when crossing the > boundary from the run-time system back to the JIT-code world, > and that was partly the motivation for special-casing > non-procedure arguments to `hash-ref'. > The biggest reason, though, was that it's more convenient to pass `#f' > than `(lambda () #f)', since a `#f' result is the most common case. > > At Sun, 18 Jul 2010 22:32:21 +0200, "Jos Koot" wrote: > > Ha, that makes sense. > > I had the (apparently wrong) idea that a thunk would be > compiled to a > > ready to use procedure and would be immediately available > at run time. > > Thanks. Jos > > > > > -----Original Message----- > > > From: robby.find...@gmail.com > > > [mailto:robby.find...@gmail.com] On Behalf Of Robby Findler > > > Sent: 18 July 2010 15:39 > > > To: Jos Koot > > > Cc: plt-scheme > > > Subject: Re: [racket] hash-ref > > > > > > It is for performance reasons. Specifically you can avoid > creating > > > the procedure over and over (if you're calling hash-ref over and > > > over). > > > > > > Robby > > > > > > On Sunday, July 18, 2010, Jos Koot > <jos.k...@telefonica.net> wrote: > > > > > > > > > > > > > > > > > > > > > > > > (hash-ref > > > > a-hash a-key a-value) > > > > In the past > > > > a-value was required to be a procedure. > > > > Now it can > > > > be anything. > > > > > > > > Nice. > > > > Two caveats > > > > though. > > > > 1. If the > > > > value of a-value happens to be a procedure to be stored, it may > > > > unintentionally be called. > > > > 2. a-value > > > > may be an expresssion that takes much computation. This > > > computation is > > > > repeated every time the hash-ref line is evaluated. > > > > > > > > Simply > > > > helped of course: I make it a habbit always to write or to > > > have macros > > > > expand > > > > to: > > > > (hash-ref a-hash a-key (lambda () > > > > a-value)) > > > > > > > > In short, I do not well understand the reason to relax the > > > contract of > > > > hash-ref and hash-ref!. > > > > Jos > > > > > > > > > > > > > > > > > > _________________________________________________ > > For list-related administrative tasks: > > http://lists.racket-lang.org/listinfo/users _________________________________________________ For list-related administrative tasks: http://lists.racket-lang.org/listinfo/users
